You may use the acfcdt/filter_value_before_update filter to modify values before they are stored in custom database
tables. The filter runs after ACF's core acf/update_value filter and does not affect values stored in core meta tables
— this purely provides a way to manipulate data stored in custom tables only. If you need to modify data consistently
across both storage mediums, consider using the
acf/update_value filter instead.
Filtering repeater field values
It's important to note that the full repeater field's payload isn't passed to to this filter. Instead, each individual field withing the repeater is passed through this filter for individual processing as ACF works through the sub field structure. If you need to filter fields within a repeater, you need to use RegEx to target individual fields due to the nature of complex field names. e.g;